Dear friend, moods swings related to menses may be part of pre menstrual syndrome, but increase in aggression and irritability after getting into relationship subsequently calling off engagement points towards underlying depression. Try taking to her about her condition , look for altered sleep pattern, appetite changes and other behavioral changes. Get her evaluated by a professional soon , medications will definitely help her out to tackle the situation. All the best.

I understand ur concern for ur sister. Mood swings before menstrual cycle can be a part of premenstrual syndrome. It is treatable with the help of medication. But mood swings affecting relationship and work requires independent assessment by a qualified psychiatrist. Seek consultation immediately. The symptoms described can be treated. Tk care.